> ## Documentation Index
> Fetch the complete documentation index at: https://docs.optimism.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Sequencer Defined Metering

> Understand how Sequencer Defined Metering lets a sequencer rebate gas, how the rebate is committed and verified, and what changes for nodes and applications at Lagoon.

export const NormativeSpec = ({what, title, href, note}) => <Info>
    <strong>Normative spec:</strong> {what} is normatively defined in the{" "}
    <a href={href}>{title}</a>. {note}
  </Info>;

<NormativeSpec what="Sequencer Defined Metering" title="Sequencer-Defined Metering specification" href="https://specs.optimism.io/protocol/lagoon/sdm.html" note="This page explains the mechanism and what it changes for node operators and applications; it does not restate the spec. The Lagoon spec is not yet finalized." />

<Warning>
  SDM is not live on any chain. It activates with the [Lagoon hardfork](/op-stack/protocol/hardforks/lagoon).
</Warning>

*Sequencer Defined Metering (SDM)* lets a sequencer charge a transaction less gas than the EVM
measured, and commit that decision in the block so every verifier reproduces it.

The gas schedule is uniform: a transaction pays the same for an operation whether or not an earlier
transaction in the same block already paid for it. SDM gives the sequencer a way to hand some of
that back. Within the bounds the spec fixes, the sequencer chooses the amounts — that is the
"sequencer defined" part. The chain still keeps a single canonical answer for what each transaction
was charged, because the rebate travels in the block as data that verifiers apply, not as a policy
they re-derive.

This page explains what the mechanism does and what it changes for the people running or building
on a chain. To turn SDM on or off as a chain operator, follow the
[SDM operator guide](/chain-operators/guides/features/sequencer-defined-metering).

## How a rebate is committed

A sequencer that issues rebates appends a single **post-exec transaction** (type `0x7D`) as the
last transaction in the block. It is not executed as code, charges no fees and consumes no gas. It
carries a payload of transaction index and gas refund pairs. If the sequencer assigns no refunds,
the block has no post-exec transaction at all.

Deposits and the post-exec transaction itself can never be rebated. For the payload encoding and
the full validity rules, see the
[SDM specification](https://specs.optimism.io/protocol/lagoon/sdm.html#validity-rules).

## What `gasUsed` means under SDM

For a rebated transaction, the EVM's measurement and the chain's accounting come apart. The
receipt's gas-used field reports the **canonical** gas: what
the EVM measured, minus the refund. That is also what the block's `gasUsed` sums. A refund can
never exceed what the EVM measured, so canonical gas is never negative.

The EVM has already charged fees on the pre-rebate figure by the time the refund applies, so SDM
re-settles the difference within the same transaction — crediting the sender and debiting the fee
recipients, atomically and without a separate receipt. The exact per-recipient amounts are in the
[spec's settlement rules](https://specs.optimism.io/protocol/lagoon/sdm.html#settlement).

## What changes at Lagoon

Before Lagoon activates, a block containing a post-exec transaction is invalid, and blocks are
produced and validated exactly as on a chain that never specified SDM. From activation, two things
become observable:

* A `0x7D` transaction may appear as the last transaction of a block, through the same
  transaction list interfaces used today.
* Receipts for standard transactions gain an `opGasRefund` field.

`opGasRefund` is a JSON-RPC field only. It is not part of the receipt's RLP encoding and not
committed to the receipts trie, so tooling that ignores unknown fields — `cast receipt` included —
is unaffected. Only consumers that opt in observe it.

Post-exec transactions are not user-submittable and do not propagate over public transaction
gossip, so mempool and transaction pool interfaces are unchanged.

## Who needs to do what

| You run                                                                     | What SDM asks of you                                                                                                                                  |
| --------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------- |
| Verifier or full nodes                                                      | Nothing beyond running software that implements Lagoon. There is no SDM configuration on a verifier.                                                  |
| A sequencer                                                                 | Software that implements Lagoon, plus an explicit opt-in. See the [operator guide](/chain-operators/guides/features/sequencer-defined-metering).      |
| An application                                                              | Usually nothing. A transaction's `gasUsed` is already the canonical amount it was charged, and `opGasRefund` is there if you want to show the rebate. |
| A block explorer, indexer, or anything else that parses blocks and receipts | Check how you handle an unrecognized transaction type before Lagoon reaches a chain you follow.                                                       |

Further detail on the last row: A block may now end in a transaction type your decoder has
never seen: `from` is the zero address, the signature fields are absent rather than zeroed, and it
emits its own receipt with everything charged set to zero. That is enough to break a strict decoder
or to surface as a malformed row. Gas totals are worth re-checking too — summing per-transaction
`gasUsed` still reconciles with the block, but it no longer equals what the EVM metered, so
anything recomputing gas independently will disagree unless it accounts for `opGasRefund`.

Rebates are sequencer-side. A sequencer that stops issuing them changes only the blocks it builds
from that point: post-exec transactions already in the chain stay valid, and verifiers keep
validating them.

## Next steps

* Turn SDM on, confirm it, or turn it off with the [SDM operator guide](/chain-operators/guides/features/sequencer-defined-metering).
* See what else ships in the [Lagoon hardfork](/op-stack/protocol/hardforks/lagoon).
* Read the normative rules in the [SDM specification](https://specs.optimism.io/protocol/lagoon/sdm.html) and the [post-exec transaction specification](https://specs.optimism.io/protocol/lagoon/post-exec.html).